1. Field of the Invention
This invention relates to stress softening of elastomeric films to improve the physical properties thereof, to articles comprising stress-softened elastomeric film, and to apparatus and methods for making stress-softened films and articles therefrom.
2. Background of the Related Art
Elastomeric films have been widely employed in the fabrication of elastic prophylactic articles, including gloves, condoms, finger cots, tubular bandages, and the like. Currently, these articles are primarily produced from a latex material via a dipping process in which an appropriately-shaped mold is dipped into a bath of the latex material, so that upon withdrawal, the mold is coated with a thin layer of the latex material. The thickness of the latex coating on the mold is dependent on the viscosity of the latex, and the speed of extracting the mold from the latex bath.
In respect of prophylactic articles, and specifically condoms, the recent spread of AIDS in the general population and the corresponding resurgence of condom usage in sexual activity has focused effort on improving the strength and reliability characteristics of condoms, and of achieving improvements in manufacturing processes and economics to further combat the spread of sexually transmitted diseases generally, and AIDS particularly, as well as to provide a safe and reliable contraceptive means.
U.S. Pat. No. 4,576,156 issued Mar. 18, 1986 to Manfred F. Dyke discloses a condom formed of a thermoplastic polyurethane material, having a generally cylindrical configuration with an open proximal end and a closed distal end. The disclosed condom has a thickness of from about 0.01 millimeters, or less, to about 0.25 millimeters. The thermoplastic polyurethane employed to form the condom is disclosed as having: an average Shore A hardness of from about 50 to about 90; a tensile stress, at 100% of elongation, between about 300 and 1,000 psi; and a tensile stress at 300% elongation, between about 800 and 3,000 psi. Suitable thermoplastic polyurethane species for manufacturing the condom include those set out at column 2, line 55 to column 3, line 10 of the Dyke patent, with polyether- or polyester-based urethane elastomers said to be preferred. In the manufacture of the thermoplastic polyurethane condom disclosed in the Dyke patent, a film of the polyurethane material, e.g., in the form of a 6-inch square, is heated to a temperature high enough to soften the polymer but low enough to avoid thermal degradation, preferably in a clamping frame, and at a temperature of about 400.degree.-500.degree. F. The heated film then is brought into contact with a preformed mandril to cause the film to assume the shape of the mandril, preferably with application of a vacuum to the system in order to bring about uniformity in wall thickness (column 3, lines 47-50 of the patent).
The film and condom article of U.S. Pat. No. 4,576,156 suffer from a number of deficiencies, which limit their utility. The film disclosed in such patent is heated to 400.degree.-500.degree. F., as necessary to soften the polyurethane-based thermoplastic elastomer without degrading the film (column 3, lines 36-41). Such temperatures will disrupt (melt) the small crystallites in the/polyurethane film which make it elastomeric. This permits the disclosed "forming operations" to be carried out. Upon cooling, these crystallites reform, but on a microscopic basis, the film is at least predominantly isotropic (column 4, line 3). Thus, the strength achieved by orienting (extruding) the film during its original manufacture is lost. Second, the vacuum forming technique disclosed in this patent is used to "ensure . . . a pinhole free device" (column 3, lines 50-52). The tip of the vacuum formed film could develop pinholes after contact with the forming mandril. Holes in such area of the film would be sealed by surrounding "softened" polymer in contact with or adhering to the mandril, and the final vacuum forming could still be effected. There is no evidence in this patent for a "substantially uniform thickness" as identified at column 4, line 54 of the patent, and such alleged uniformity of thickness is contrary to what reasonably would be anticipated from the mode of fabrication employed for forming the disclosed condom article. The thickest portion should be the tip, and the thinnest portion should be the region where the hemispherical end of the condom meets the cylindrical sheath thereof. The heated fabrication described in this patent is fundamentally different in procedure and result from room temperature fabrication. The deformation effected by the mandril and vacuum causes the film to conform to the mandril. Thus, based on the method of fabrication disclosed in the patent, the resulting product article would appear to be characterized by substantial variation in film thickness from along the cylindrical sheath to the distal end of the condom.
U.S. Pat. No. 4,735,621 to L. Hessel discloses a tubular protective condom-like device comprising a flexible, thin-walled tube that may be formed of polyurethane. The tube is closed at one end and has at its opposite, open end, a collar-shaped, outwardly extending portion with means for radially stretching the open end. In one disclosed embodiment, the device has a first outwardly extending ring-shaped means adapted for radially extending the open end, and a second outwardly extending ring-shaped means that is adapted for radially extending the closed end. The second ring-shaped means thus secures or maintains the device in the vagina in manner similar to a diaphragm. The inner diameter of the device is sufficiently large to permit movement of a penis during coital contact.
U.S. Pat. No. 4,603,174 issued Jul. 29, 1986 to T. Okada, et al for "STRETCHED POLYPROPYLENE FILM", describes a stretched polypropylene film with allegedly excellent see-through characteristics, which is obtained without degrading the inherent properties or film-forming character of the polypropylene resin. The film is formed from a melt of a polypropylene resin containing a specific alpha-olefin and/or vinyl cycloalkane, and then stretched in at least one direction. The olefin and vinyl cycloalkane materials are described in the paragraph bridging columns 1 and 2 of the patent. The stretching techniques are described at column 2, lines 55-60 as employing conventional industrial methods, e.g., roll stretching, tenter stretching, and tubular stretching, in at least one direction, at a stretch ratio of 1.2 to 100 times in terms of the area stretch ratio.
In the paragraph bridging columns 2 and 3 of the patent, the benefits of stretching are described:
"Needless to say, a stretched film having excellent see-through characteristics without optical nonuniformity can be obtained by stretching polypropylene resin sheet used in this invention. It has also been ascertained that as an incidental effect, the polypropylene resin sheet has better stretchability than conventional polypropylene resins. Specifically, the present inventors observe the reduction of the stretching stress and the decrease of the film breakage phenomenon during stretching." PA1 "[T]hese ratios may also be related to the degree of molecular orientation in the product, and, since this has a marked effect on certain physical properties of the products, in particular their moduli, it follows that the use of our process can enable extruded and/or drawn articles (other than those of dimensions previously excluded) to be produced having moduli higher than those previously attainable. Thus a further aspect of our invention resides in such products as novel articles."
Example I of the patent describes the polypropylene sheet being stretched by a tenter-type consecutive biaxial stretching device to four times in the machine direction at 145.degree. C. and subsequently to ten times in the transverse direction at 160.degree., and then heat-treating at 145.degree. C. to obtain a biaxially stretched file having a thickness of about 20 microns.
U.S. Pat. No. 3,637,579 issued Dec. 14, 1971 to C. J. Heffelfinger for "UNIDIRECTIONALLY ORIENTED FILM STRUCTURE OF POLYETHYLENE TEREPHTHALATE", describes a substantially unidirectionally-oriented polyethylene terephthalate film which among other characteristics has at least 50% elongation at break in the direction transverse to the direction of predominant orientation. The molecular orientation of the film is predominantly uniaxial by stretching at least four times the original dimension of the film in the direction of stretch. The film has a tensile strength of preferably at least 50,000 psi in the direction of stretching, and at least 50% elongation in the direction transverse to the stretching.
The patent discloses that stretching a film of polyethylene terephthalate is desirable from the standpoint of greatly increasing the tensile strength of the film in the direction of stretching. Stretch ratios of from slightly greater than one-fold to about five-fold are described (column 2, lines 68-70). In the sentence bridging columns 2 and 3 of the patent, it is disclosed that film stretching is desirable, by virtue of the fact that a film stretched two-fold possesses a modulus of 460,000 psi, whereas a film stretched five-fold possesses a modulus of about 1,800,000 psi. A film stretch ratio greater than five-fold is said to result in the failure of the film structure by fibrillation "of such an extensive nature as to destroy completely the useful structural integrity and unitary structure of the film" (column 3, lines 8-11). The unidirectionally stretched film is said to have an unexpected and totally surprising pneumatic impact strength (column 4, lines 15-16).
U.S. Pat. No. 3,733,383 issued May 15, 1973 to J. B. Bunney, et al, for "DEFORMATION OF POLYMERIC MATERIALS", describes a process for reducing the cross-sectional area of an article of an orientable thermoplastic polymeric material by drawing the article at a temperature below its melting point through a die of smaller cross-sectional area than that of the article. The deforming surface of the die is well lubricated and the molecular orientation of at least that part of the article to which drawing tension is applied, is such that the tensile strength of the article exceeds the drawing tension. The term "article" is defined at column 1, lines 52-57 as excluding articles having a cross-sectional area less than 0.01 square inch and whose largest external dimension is less than 0.05 inch. At column 2, lines 48-50, the patent refers to relaxation of extruded thermoplastic materials, due to hydrostatic extrusion effects. The process described in the patent is said to reduce the extent of such relaxation. At column 4, lines 34-42, the patent, in referring to deformation ratios applicable to extrusion of thermoplastics, states that:
At column 1, lines 1-36, the patent discloses various X-ray diffraction patterns which are said to indicate very high degrees of molecular orientation, which in turn is said to be the reason for increased modulus in products of the invention.
U.S. Pat. No. 3,157,724 issued Nov. 17, 1964 to I. O. Salyer, et al, for "PRODUCTION OF HIGH STRENGTH ORIENTED ETHYLENE/VINYL ACETATE COPOLYMER FILM", describes the orientation of a film of high molecular weight ethylene/vinyl acetate. The orienting process is carried out at elevated temperatures which are generally above 25.degree. C., but considerably below the melting point of the polymer. The copolymer is stretched at a rate of 50% to 5,000% per minute, and is stretched almost to the breaking elongation of the copolymer (e.g., 70%-90% of break) under conditions suitable for effecting and maintaining orientation. The oriented ethylene/vinyl acetate film of the invention is said to be remarkably improved in tensile and impact strengths, as well as uninitiated tear strength.
U.S. Pat. No. 3,247,857 issued Apr. 26, 1966 to M. S. Kanbar for "DENTAL FLOSS", describes a dental floss made of "exceptionally soft material, the floss having a smooth surface and a high tensile strength whereby the teeth and gums may be cleaned efficiently and without injury thereto" (column 1, lines 8-12). This improved dental floss is formed by loosely twisting a tape of an oriented polymer, such as a polyethylene or polyamide, into a helix forming a small compressible tube (column 1, lines 35-38). The starting material may be smooth film formed from an extruded polymer such as "Saran", polyethylene, polyesters, or polyamides, at a melt extruded film thickness of approximately 0.00015 inch. This film is slit into a tape of approximately 1 inch width, or alternatively, the film is extruded from a die in the desired tape width. The tape is stretched by passage through rolls comprising feed rolls and drawing rolls operating at different speeds, so that the tape is stretched therebetween. Stretching of the tape is said to orient the molecular structure of the film and thereby increase its tensile strength. The drawing may be hot drawing or cold drawing. The tape is stretched to approximately three times its initial length to produce a ribbon whose width is about one-half that of the original tape and about half as thick. The patent notes that after this treatment, "[t]he ribbon is much stronger than the stretched tape and is very soft" (column 2, lines 4-5). The ribbon then is twisted approximately 1-3 turns per inch to produce a floss having the desired character.
U.S. Pat. No. 32,983 reissued Jul. 11, 1989, to S. B. Levy, for "BALLOON AND MANUFACTURE THEREOF", discloses a polymeric balloon. suitable for use in a balloon catheter system. The patent describes forming the balloon at a temperature of preferably 84.degree. C.-99.degree. C. by drawing a polymeric, preferably a polyethylene terephthalate homopolyester, tubing having an internal diameter which preferably is about one-half the outer diameter, to a length which is approximately three-fold to six-fold the original length. Thereafter the drawn tubing is expanded to an inner diameter which is preferably six-fold to eight-fold and an outer diameter which is preferably about three-fold to four-fold times the original inner and outer diameter, respectively. The balloon has a burst pressure of at least 200 psi and a radial expansion beyond nominal inflated diameter of less than 5% at 200 psi. The tubing material has an intrinsic viscosity of 0.8 to 1.1, and is formed by conventional extrusion techniques from PET homopolyester resin. The tubing is expanded in a confining apparatus shown in FIG. 1 of the patent, by means of a fluid such as nitrogen gas. The patent does not describe any hardness, or modulus, characteristics being changed as a result of the tubing expansion to form the balloon.
U.S. Pat. No. 3,304,353 issued Feb. 14, 1967 to A. Harautuneian for "METHOD OF CATHETER MANUFACTURE", describes a method of manufacturing balloon-type catheters constructed "entirely from plastic materials devoid of possible irritants such as rubber compounding or curing agents, all in a manner such that the balloon constitutes, in effect, a terminally integrated surface continuity of the tube of the necessary properties for sustained inflation" (column 1, lines 33-39). The patent at column 1, lines 47-51 describes illustrative balloon materials of construction as including elastomeric grades of polyurethane such as Goodrich "Estane" or Mobay Chemical "Texin". More particularly, the balloon layer is formed of a poly(esterurethane) of elastic grade and having a 300% stretch modulus within about the 600 to 1200 psi range. The catheter comprises a tube with a main passage and a second passage having a hole in its outer wall communicating with the exterior. The opening is covered with a coating of a water soluble partitioning material. Thereafter, the coating is covered with a thermoplastic balloon layer. Subsequently, in use, liquid injected through the second passage functions to dissolve the partitioning coating to establish fluid communication with the balloon layer, so that the balloon can be inflated to the desired extent. The poly(esterurethane) layer is applied at thickness of from about 0.003 to about 0.008 inch by dipping the tube in poly(ester-urethane) resin dissolved in a suitable solvent such as THF, dimethyl formamide, or an 80-20 solution of THF and cyclohexane.
U.S. Pat. No. 4,833,172 issued May 23, 1989 to R. A. Schwartz, et al, for "STRETCHED MICROPOROUS MATERIAL", describes a method for producing stretched microporous material. A sheet is formed from a mixture comprising essentially linear ultra-high molecular weight polyolefin, e.g., polyethylene having an intrinsic viscosity of at least about 18 deciliters/gram, polypropylene having an intrinsic viscosity of at least about 6 deciliters/gram, or a mixture thereof, together with finely divided particulate substantially .water-insoluble siliceous filler, and a processing plasticizer which is a liquid at room temperature. The processing plasticizer is substantially removed from the sheet to form a precursor microporous material, and the precursor microporous material is stretched in at least one stretching direction to at least one stretch ratio of at least about 1.5. This produces stretched microporous material which is dimensionally stable at room temperature, and has a stretch ratio in the stretching direction of at least about 1.5. The matrix of the stretched microporous material comprises (i) regions of stretched molecularly oriented ultra-high molecular weight polyolefin distributed throughout the matrix of the stretched material, (ii) filler distributed throughout the matrix of the stretched material, and (iii) a network of interconnecting pores communicating throughout the stretched microporous material. The patent at column 10, lines 25-48 discusses uniaxial stretching as well as biaxial stretching.
U.S. Pat. No. 4,867,937 issued Sep. 19, 1989 to H-M Li, et al, for "PROCESS FOR PRODUCING HIGH MODULUS FILM", describes a two-step process for increasing the modulus of a film in one or both directions. A thermoplastic film is drawn in at least one direction in a medium with a temperature of between about 10.degree. C. above the glass transition temperature and 40.degree. C. below the melting temperature, at a draw ratio of between 1.05 and 5.5, and is drawn in the same at least one direction in a medium having a temperature between about 5.degree. C. and 35.degree. C. below the melting temperature, at a draw ratio of between about 1.05 and 2.5. The films may be formed of materials such as those illustratively mentioned at column 2, lines 36-44 of the patent. The film-forming polymers are heated into a melt and then extruded through a nozzle or a die slit to form a film which then is cast on a cooling drum and solidified. The cast film is drawn a first time in either the longitudinal and/or transverse directions through a medium such as air at the requisite temperature, in which the drawing increases the tensile modulus of the film in the directions(s) of draw, followed by a second drawing step which induces substantial increase in the tensile modulus in at least one direction, thereby providing a 20%-40% improvement in the tensile modulus in the longitudinal and/or transverse direction.
U.S. Pat. No. 4,855,169 issued Aug. 8, 1989 to M. W. McGlothlin, et al, for "PROPHYLACTIC SHEATH WITH AUGMENTED BORDER", describes a prophylactic sheath fabricated from an elastic polymer material which is augmented along the border at the open end with a resilient material having a 100% tensile modulus substantially lower, preferably lower by at least 75%, than the modulus of the sheath material. Such provision is said to facilitate rolling of the edge and stretching of the sheath for purposes of application, without compromising the high degree of sensitivity in terms of heat and sensation transmission associated with the thin-walled sheath. The prophylactic may suitably be formed of thermoplastic elastomer materials, such as polyurethane, or block copolymer thermoplastic elastomer materials. The condom is described as being made by dip molding of a form or mandrel, and applying the augmented border in the form of a ring or band to the open end of the sheath, either removably or permanently. Bonding of the border element is achieved by self-curing, adhesives, or other conventional techniques such as fusing by heat or solvents. The border may be formed of a closed cell foam or solid elastomer. A wide variety of bands and rings are described as embodiments of the invention, together with a number of methods of forming same. The materials of construction of the augmented border are illustratively set forth at column 7, lines 23-42, and include various block copolymers such as Kraton.RTM. copolymers and polyurethanes, with foam polyurethanes being most preferred. In the paragraph bridging columns 8 and 9 of the patent, it is stated that "[t]he polyurethane used for the sheath will be one which combines high strength with a high degree of softness." The tensile strength is at least about 6,000 psi. The softness, expressed as Shore A hardness, preferably ranges from about 50 to about 90, most preferably from about 60 to about 80. The sheath thickness is generally less than about 1.4 mils, and preferably is from about 0.4 to about 1.4 mils, and the 100% tensile modulus is at least about 200 psi.
U.S. Pat. No. 4,817,593 issued Apr. 4, 1989 to R. A. Taller, et al, for "PROCESS FOR PREPARATION OF POLYURETHANE CONDOMS", describes a dipping method for making polyurethane condoms, using a solvent solution of a polyurethane polymer or prepolymer which is the reaction product of a polyisocyanate with at least one longchain polyol having an average molecular weight of from about 500 to about 5,000 and a hydroxy number of about 225 to about 22.4, with an NCO/OH ratio of from about 0.95/1 to about 1.1/1. The condoms of this patent have a 100% tensile modulus of less than about 150 psi, and a thickness of between about 1.5 and about 4.0 mils. The polyurethane polymers used in the condom have a Shore A durometer hardness of about 35 to 60 (column 5, lines 4-5). The patent in the paragraph bridging columns 4 and 5 thereof describes the polyurethane polymers employed in the condom as having hard segments and the degree of cross-linking as being balanced within the ranges of approximately 14% to 25% hard segments and approximately 5,000 to 30,000 molecular weight per cross-link (M.sub.c). The patent describes the advantages of low modulus materials of construction employed in the disclosed condom. Chemical cross-linking and physical cross-linking are employed in the polymer to reduce crystallization to yield the desired low modulus (column 5, lines 28-47).
U.S. Pat. No. 4,808,174 issued Feb. 28, 1989 to R. Sorkin for "CONDOM OF PLASTIC MATERIAL", discloses a condom of plastic material, preferably selected from the class which includes polyethylene, polypropylene, and vinyl materials. The condom also includes a pubic shield integral with it which is thicker than the material along the tubular length of the condom, with an adhesive preferably being applied to the pubic shield for attachment during coitus. The condom is described as being formed with a preferably translucent, and relatively thin plastic material in the range of about 0.03 millimeters. The patent discloses to reinforce the condom with fibrous material such as elongate strands of fine diameter fibers. The plastic shield is a circular disk of about 5 inches diameter and is of integral construction with the condom. There is no disclosure in the patent of strength or modulus properties of the materials of construction employed in the disclosed condom.
U.S. Pat. No. 4,881,553 issued Nov. 21, 1989 to R. A. Grossman for "MESH REINFORCED CONDOM", describes a condom which comprises a latex sheath having a reinforcing elastic mesh embedded in the walls of the sheath, with the elasticity of the mesh being about equal to or less than the elasticity of the latex sheath. The mesh may be coextensive with the length of the sheath or may be embedded in only the upper one-half or upper one-third end of the Sheath. Column 2, lines 13-16 of the patent describes the mesh elasticity as being on the order of about 5% to 75% less than the elasticity of the condom walls. The mesh is stated to be formed from "an elastic thread such as natural rubber thread, or thread formed of synthetic rubber, silicone elastomer, or a fiber/polymer blend such as cotton/rubber. The wall of the condom disclosed in this patent is approximately 0.02-0.09 millimeters in thickness. The patent at column 3, lines 10-14 discloses that "when the reinforcing mesh is made from a flat sheet of thin plastic film, the projections 16 can be either embossed on the plastic film or formed by flaps of the film between slits 22."
U.S. Pat. No. 4,964,416 issued to Robert G. Wheeler discloses a variety of condoms which are amenable to construction from thermoplastic elastomeric materials including polyurethane materials, polyester elastomers, polyether block amides, etc.
In general, the use of synthetic elastomeric materials afford substantial advantages over latex as materials of construction for prophylactic articles. The strength and tensile modulus of polyurethane elastomers are about 3 times those of latex, an advantage that is also found to a greater or lesser extent in other thermoplastic elastomer materials, especially block copolymers comprising alternating hard and soft segments. In addition, thermoplastic elastomeric materials can be employed to form prophylactic articles of equivalent strength at substantially reduced thicknesses, relative to corresponding latex rubber articles. Further, synthetic thermoplastic elastomer materials display much greater chemical inertness to lubricants, spermicides, and the like, to which latex rubber may be susceptible to degradation or attack, particularly when these materials are petroleum-based in character.
A major deficiency of such thermoplastic elastomeric materials, however, is their stiffness and relatively high modulus character, and their lack of high elasticity, relative to latex rubber materials. In particular, the soft, supple, highly flexible character of latex rubber films, and their smooth, textural characteristics (referred to as "hand") generally are not matched by films of thermoplastic elastomeric materials.
Handbook of Thermoplastic Elastomers, Second Edition, B. M. Walker, et al (Van Nostrand Reinhold Company, 1988), pages 15 and 16, discusses the morphology of styrenic block copolymers, and shows changes in morphology of an A-B-A block copolymer, as a function of composition, so that with increasing content of A, the morphology changes from (1) A spheres in B continuous phase at low concentration, to (2) A cylinders in the B continuous matrix, followed by (3) A,B lamellae, with still further increasing A content producing (4) B cylinders in continuous A matrix, and finally (5) B spheres in continuous A matrix. The text states that these block copolymers, particularly those with a continuous polystyrene phase, show obvious stress softening, i.e., when the polymer is stretched to an elongation below its ultimate elongation, allowed to retract, and is then restretched, it appears much softer during the second extension than during the first. This feature is described as being similar to the so-called "Mullins effect" in conventional reinforced vulcanizates, and appears to be caused by the rupture of the continuous polystyrene phase during stretching to yield discrete domains.
Other references relating to softening of thermoplastic elastomeric films by application of stress followed by relaxation of same include: "Thermodynamics of The Deformation of Segmented Polyurethanes With Various Hard Block Contents. (II. Stress Softening and Mechanical Hysteresis)," Godovsky, U. K., et al, Colloid Polym. Sci., Vol. 267, No. 5, pages 414-420 (1989)) (thermodynamics of stress-softening and hysteresis in polybutadiene polyurethanes, and adverse impact on industrial application of polyurethanes by stress-softening and hysteresis losses); "Effect of Casting Solvent on the Stress-Hardening and Stress-Softened Characteristics of Kraton-G 1650 Copolymer Films," Cowie J. M. G., et al, J. Macromol. Sci., Vol. B 16, No. 4, pages 611-623 (1979) (stress softening of Kraton-G cast from N-heptane, and subjected to repeated stress cycles); "Mechanical Properties of High-Density Cellular Urethanes," Payne A. R., et al, J. Elastoplastics, Vol. 5 (July 1973), pages 161-177 (hysteresis and stress softening of urethanes is recoverable up to temperatures of 170.degree. C.); "Infrared Studies of Segmented Polyurethane Elastomers (II. Infrared Dichroism)," Estes, G. M., et al, Macromolecules, Vol. 4, No. 4, pages 452-457 (1971) (stress hysteresis of urethane domains of polyether- or polyester-polyurethanes); and "Stress Softening in Elastomer Blends," Meluch, W. C., Journal of Appl. Poly. Sci., Vol. 13, pages 1309-1318 (1969) (stress softening of ternary elastomeric system of natural rubber, nitrile rubber, and brominated butyl rubber (Hycar.RTM. 2202)).
It is important in understanding the above-described references and the field of the invention to distinguish between the useful stress softening of films, condoms, diaphragms, gloves, and other articles of thermoplastic elastomers, on the one hand, and the strengthening and stabilizing of films and filaments of semicrystalline thermoplastics, on the other hand. Distinctions can also be made between stress-softening and the heat-stretching of thermoset polyurethane filaments, e.g., those commercialized under the trademark Spandex.RTM..
Most of the work reported on stress-softening points out the substantially permanent set that occurs in the stressed material. Indeed, stress softening is used as an explanation of what happens when a thermoplastic elastomeric article is subjected to stress, undergoing an undesirable deformation and becoming permanently deformed. This limits the utility of such articles. The art has not recognized the benefits of stress softening, and in fact this phenomenon has heretofore been considered troublesome.
The stress-softening of TPE films differs from the stretching of Spandex.RTM. yarns during manufacture. The polyurethane dopes (solutions of polyurethanes) are spun into filaments by dry (evaporation of solvent) or wet (coagulation of solution in a bath) processes. The polyurethane filaments then are subjected to stretching and heating to cause further reaction and setting. The resulting yarns are stable (to hot water during dyeing, for example, and to laundering). Without such treatment, such yarns would undergo permanent deformation (elastic bands in garments would become loose, for example). The tensile modulus, however, increases. This is not stress-softening.
Accordingly, it is an object of the present invention to provide thermoplastic elastomeric films which are stress-softened in character and amenable to usage in a wide variety of articles comprising such films.
It is another object of the present invention to provide articles comprising films of the foregoing type, as well as a method of making same.
It is yet another object of the present invention to provide an improved condom article formed of a thermoplastic elastomeric film material, which is stress-softened in character.
It is a still further object of the present invention to provide a method and apparatus for making such stress-softened thermoplastic elastomeric films and articles.
Other objects and advantages will be more fully apparent from the ensuing disclosure and appended claims.